Please add the tests directory to the sdist tarballs

[eli-schwartz]

Downstream users may wish to run the tests to make sure everything works on their version of python / dependencies.

[brunns]

Travis runs the tests on all the versions which PyHamcrest supports - see https://travis-ci.org/github/hamcrest/PyHamcrest. Do people need this?

[offbyone]

Resolved in #150 and will be included in the next release